The present invention relates to integrated circuits and more particularly, to a single supply digital voltage level shifter.
Many of today's integrated circuits include sub-circuits or modules that operate at different voltage levels. Sometimes these modules are required to interface with one another, thereby requiring the voltage level of one circuit to be “shifted” with respect to the voltage level of the other circuit. Thus, there are transitions from one logic level to another during operation of the circuit. These transitions typically are between a logical high state at some voltage above ground level, and a logical low state at ground level. For example, the voltage in one module may have a logic high voltage level of 0.75V and the voltage in the other module may have a logic high voltage level of 1.32V. The first module has a voltage swing of zero to 0.75V and the second circuit has a voltage swing of zero to 1.32V. Level shifters provide the connection between two such circuits, shifting the level of the signals from the first voltage swing to the second voltage swing.
In conventional cross-coupled level shifter architectures, the need for two voltage supplies requires that the level shifter must be of a minimum physical size, a constraint which led to the development of the single supply level shifter, such as the conventional single supply level shifter 100 illustrated in FIG. 1. The single supply level shifter 100 requires only one supply VDDH to convert a low voltage signal VDDL at the input “in” to a higher voltage VDDH. The consequential reduction in foot print realized by the removal of one of the voltage supplies, and the simplification of pin placement and wire routing leads to increased flexibility in where such level shifters can be placed.
The threshold drop across the N-type MOSFET n1 provides a virtual VDDL (the lower voltage of the input signal) to the input inverter p2, n2. However, this conventional design is at a significant disadvantage in that higher leakage currents can occur if the input signal supply level is lower or VDDH is higher than the input supply level by more than the threshold voltage of n1. Therefore, the level shifter 100 is limited in its range of input signal or power supply. Also, the diode-connected transistor n1 limits the operating speed of the level shifter 100.
Thus, there is a need for a single supply digital voltage level shifter that provides some improvement on the above-mentioned problems.